1. Technical Field
The invention relates to an electrophotographic photoreceptor, a process cartridge, and an image forming apparatus.
2. Related Art
Recently, electrophotographic methods have been extensively applied to image forming apparatus, such as photocopiers or a printers. Since an electrophotographic photoreceptor (hereinafter, occasionally referred to as ‘photoreceptor’) used in an image forming apparatus using an electrophotographic method is exposed to various types of contacts or stresses in the device, deterioration thereof may occur. Meanwhile, high reliability is required in conjunction with digitalization or colorization of image forming apparatus.
Among such photoreceptors, currently, organic photoreceptors are extensively used. Organic photoreceptors are inexpensive in comparison with photoreceptors including amorphous silicon, and are safer than photoreceptors including selenium or cadmium sulfide. However, since organic photoreceptors have low hardness as compared to photoreceptors including selenium or cadmium sulfide, if an organic photoreceptor is repeatedly used in an image forming apparatus, abrasion may occur due to friction with a cleaning member, a developer, or the like. If the photoreceptor is abraded, problems occur such as reduced lifespan and the need for short cycle replacement. Additionally, since surface roughness is increased due to the friction, slidability may deteriorate.
In order to solve such problems, an approach of forming a hard inorganic material as a protective layer on an organic photoreceptor has been adopted. Examples of materials of such protective layers under investigation include amorphous carbon (diamond-like carbon), oxides, nitrides and nitrogen oxide, which are hard and relatively high in electric resistance. Among such inorganic materials, the present inventors have already found that thin films composed of oxygen and gallium possess both wear resistance and image maintenance properties.
In the above technique, it is desirable that the protective layer has a larger thickness in view of durability. When the protective layer has an increased thickness, the halftone concentration of output images may be greatly decreased when used repeatedly. In such a case, photoreceptors exert electric properties characterized by high residual potential, and there is the problem that a high residual potential results in fluctuations in concentration during repeated usage.